Add shellcheck CI

Trying to set up a shellcheck CI as proposed in #1 (closed). I'm not sure this is correct. It's mainly copy-paste and reduced to the minimum.

The .ci/shellcheck.sh is taken from pmbootstrap (source). I took only the last part of that "shellcheck.sh" and also skipped the "Description" in the header (more minimalistic but possibly I should add it again, not sure). In the "shellcheck.sh" of pmaports, the command is executed with options -e SC1008 -x (source), maybe this should be added here too?

The .gitlab-ci.yml is taken from pmaports (source). I skipped the dashes --- at the beginning and the end, reduced to "global settings" and "shellcheck". On "global settings" I removed the .ci/move_logs.sh (not sure on this) and the stage "build". Something I don't know: How the CI finds the image "alpine:latest".

The mix of pmabootstrap and pmaprts is because of:

• The current .ci/shellcheck.sh of pmaports (source) reads and partially creates a list of files and then works through this list. The "shellcheck.sh" in pmbootstrap looked more appropriate for the small "swclock-offset" repository.
• There is no current .gitlab-ci.yaml in pmbootstrap but a .build.yml (source) because of the move to sourcehut. The old .gitlab-ci.yml of pmbootstrap (source) looks similar to the one in pmaports but doesn't contain a "shellcheck".

Cc: @Newbyte @ollieparanoid

Closes #1 (closed).